• Experimenters try to control extraneous factors that can affect the dependent variable and threaten the validity of their conclusions.
  
  
• Confounds threaten the internal validity of a study. Experimental confounds are nuisance variables correlated with the independent variable.
  
  
• Order effects threaten the internal validity of repeated-measures designs.
  
  
• Differential mortality is a common threat to validity in mixed factorial designs.
  
  
• Positive self-presentation, Hawthorne effects, and expectancy bias can all threaten construct validity.

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